1. Field of the Invention
The present invention operates in the field of gas chromatography with capillary columns and more particularly refers to the introduction of large volume liquid samples directly into a capillary column (on-column injection) for subsequent analysis of the same in a known gas chromatography apparatus.
The introduction of large volume liquid samples (samples are formed by the substances to be analyzed strongly diluted in a proper solvent) allows analysis of compounds at very low concentrations or with very small quantity of substance, since the whole sample can be injected, even if problems can arise due to the large quantity of solvent introduced.
"Large volume" as here used means a quantity of sample which is relatively large compared to the volume conventionally injected with splitless or on-column modes. A precise definition of large volume doesn't exist, but generally, the introduction of a sample larger than 3 .mu.l in columns of small inside diameter (0,2 mm or less) and larger than 5 .mu.l in columns of bigger diameter is considered "large volume".
Large volumes of sample are introduced following the known on-column technique or vapor splitting technique named Programmed Vaporizing Temperatures (PTV) in the "solvent splitting" mode.
The present invention relates to the on-column technique.
2. Description of the Prior Art
To accelerate the unloading of the solvent vapors, a solvent vapor exit duct is placed in derivation on a gas chromatography column-pre-column connection.
This duct is kept open for the time necessary to eliminate most of the solvent vapors and then is closed to send the remaining part of solvent and the substance to be analyzed into the gas chromatography column, according to a well known methodology.
The main problem of these techniques arises from the need to unload the solvent vapors in such a way that the components of the substance remain at the entry of the column.
To solve this problem two different methodologies have been developed that are distinguished by their behavior towards the volatile element and are called respectively:
Concurrent solvent evaporation, and PA1 Partially concurrent solvent evaporation, which is achieved with the help of a pre-column and the re-concentration of the sample by means of the "retention gap" technique. PA1 by carrying out a direct introduction (on-column) of large volumes of sample in an entry section of the column sufficiently short to prevent a widening of the peaks due to excessively long initial bands, PA1 by carrying-out a direct introduction (on-column) of large volumes of sample in a section of column or in a pre-column having a length of 0,5-2 m that can be coated with stationary phase if required, to take advantage of an higher inertia, and PA1 by achieving a high retention power of the volatile components of the sample by means of the formation of a layer of liquid sample having a limited length.
The first methodology (concurrent evaporation) provides for the introduction of the sample directly into a pre-column using such injection conditions to obtain that all the solvent evaporates during its introduction; in particular, the injection rate of the solvent into the pre-column must be equal or less than the vaporization rate of the solvent in the pre-column.
This concurrent solvent evaporation technique provokes losses of components of the sample that are eluted at oven temperatures under around 120-150.degree. C. (if injected in volumes greater than 100 .mu.l and dissolved in a volatile organic solvent). Its field of application is therefore narrow. On the other hand, the definition of operational conditions is simpler, the pre-columns could be short (typically 1-2 m) and virtually limitless volumes of sample could be injected. For this reason, it is the preferred technique for in line LC--GC (liquid chromatography--gas chromatography) when the components to be analyzed permit its use.
Therefore, concurrent solvent evaporation is used mainly in LC--GC chromatography joined in line, where the LC fractions, normally of 200-1000 .mu.l, are introduced in GC. According to the state of the art, the introduction is mainly effected by means of a loop type interface. The plug of liquid sample is made to advance by the carrier from a loop of sample within an uncoated (no stationary phase) pre-column of 1-3 m.
The temperature of the pre-column, or more exactly of the oven in which the pre-column is lodged, is selected so that the vapor pressure of the solvent at the front end of the liquid sample stops the flow and prevents it from penetrating in depth into the column.
The advantage of the introduction through the loop is that the introduction rate is self regulated. The disadvantage, however, is often of a violent movement of the liquid forward and back for a distance that easily extends for 1,5 m. The components of the sample are then deposited along all the length of the wet zone.
As an alternative, the concurrent solvent evaporation is achieved by means of introduction of the sample at a controlled rate. The carrier is switched-off and a pump pushes the liquid sample to the entry of the column in the GC oven. The oven temperature is selected in such a way as to be sufficiently elevated to create a solvent vapor pressure that unloads the vapors of solvent from the introduced liquid. Both the techniques are called "vapor overflow" because there is no flow of carrier gas during the introduction of the sample.
The methodology called "partially concurrent solvent" evaporation (with retention gap) provides that the flow of carrier gas is active during the introduction and that the evaporation rate is lower than the injection rate, so that at least part of the sample is distributed as a liquid in the entry of the column, forming an expanding wet zone. The entry of the column consists usually of a relatively large uncoated pre-column.
The liquid sample at the entry of the column is used to trap the volatile components of the sample in the solvent (solvent trapping). This allows the solvent to vaporize and unload without loosing the solutes. The solutes are re-concentrated at the entry of the analytical column and the chromatographic process begins. Solvent trapping uses the liquid sample as a temporary gas-chromatographic stationary phase to hold back the volatile components of the sample. Since the sample film has a thickness of the order of 10-30 .mu.m, its retention power is high. To be effective, the liquid sample layer must be downstream of the evaporation point, so that the components contained in the unloaded solvent vapors can be extracted and held back. The solvent trapping has been described in K. Grob, in: "On-Column injection in Capillary GC," Huethig, Heidelberg, 1987, 1991.
Because of the entrapment by the solvent, the technique of partially concurrent solvent evaporation (retention gap) has a much wider field of application than the technique of concurrent evaporation: virtually all the components of the sample eluted after the solvent can be analyzed quantitatively. Since the zone wetted by the sample is long, it is necessary to use an uncoated pre-column to tighten the initial bands of the retention gap effect. It must be relatively long (typically 10 m.times.0.53 mm i.d.), and this could constitute a drawback for the analysis of given types of components.
According to the conventional theory, the liquid sample is injected at a rate greater than the vaporization rate, so that the liquid sample undergoes to an expansion in the column (retention gap with solvent trapping technique), or the sample injected at a lower rate so that a concurrent evaporation without solvent trapping is obtained. Therefore, always according to the traditional technique, the solvent trapping seems to be tied to the liquid that expands in a pre-column.